Transformation of the state of polarization (SoP) of light beams is important in many applications. For example, fiber optic communication systems often include optical devices that are sensitive to the direction at which light is polarized. Yet typical optical fibers do not preserve the state of polarization. Further, the different polarization directions can have different propagation velocities (birefringence) and thus can contribute to the degradation of optical communication signals. To counter this deleterious effect known as polarization mode dispersion, a polarization transformer can be used to actively adjust the polarization state of the incoming light signal so that a corrective different delay can be applied and thereby correct for the velocity mismatch.
Much progress has been made in the development of polarization transformers, but current devices are not very satisfactory for many applications. In many applications, it is desirable that such devices have fast response time and low insertion loss. The current devices include integrated optical waveguides, liquid crystal-based devices and optical fiber-based devices.
The integrated optical waveguides are typically made of lithium niobate and can include an input phase shifter section, a polarization converter section and an output phase shifter section. These waveguides have very fast response times; however, such devices have high insertion loss and are expensive to produce.
The liquid crystal-based devices include nematic liquid crystal cells whose birefringence is controlled using an external voltage. The liquid crystal devices are slower than the lithium niobate waveguides and are difficult to control in intermediate states. Although it has the desirable features of low insertion loss and low required operating voltage, the long term reliability of organic materials and the relatively low switching speed of a liquid crystal-based device are not suitable for many applications.
The optical fiber based devices use a fiber loop arrangement which operates according to the principle that changing the relative plane of the fiber loops changes the polarization state at the output. The fiber loop devices have very low insertion loss but are the slowest of the three current device types.
There is a need for an approach to fabrication of polarization transformers which provides devices that have very fast response time and low insertion loss.
Accordingly, an embodiment of a polarization transformer comprises at least one plate of transparent polycrystalline material which has an optical axis oriented perpendicular to a propagation direction of incident radiation having a first polarization state. The at least one plate includes electrodes for applying an electric field across a plane of the plate perpendicular to the propagation direction so as to provide controlled phase change such that the polarization of radiation transmitted through the polarization transformer is transformed from the first polarization state to a second polarization state. The plate in a preferred embodiment comprises a ferroelectric complex oxide such as lead lanthanum zirconate titanate (PLZT) material. Such a material provides devices that have very fast response (on the order of microseconds) and low insertion loss.
According to an aspect of the material, the plate is isotropic in the absence of an applied electric field across the plate electrodes. An electric field corresponding to an applied voltage less than 400 volts provides a phase retardation of about 180 degrees.
In accordance with another embodiment, the polarization transformer comprises first and second plates disposed such that the incident radiation is transmitted through the two plates and the orientation of an optical axis of the second plate is at an angle such as 45 degrees with respect to the optical axis of the first plate.
In accordance with yet another embodiment, the polarization transformer comprises first, second and third plates disposed such that the incident radiation is transmitted through the three plates and the orientation of an optical axis of the second plate is at 45 degrees with respect to the optical axis of the first and third plates.
According to an another aspect, the polarization transformer is capable of transforming a first arbitrary polarization state at its input to a second arbitrary polarization state at its output.
A preferred embodiment of the invention includes a polarization transformer in an optical communication system including an input optical fiber, an input optical coupling, a polarization transformer, an output optical coupling and an output optical fiber.